Ciliate

The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group (although the peculiar Suctoria only have them for part of their life-cycle) and are variously used in swimming, crawling, attachment, feeding, and sensation. Ciliates are an important group of protists, common almost anywhere there is water — in lakes, ponds, oceans, rivers, and soils. About 3,500 species have been described, and the potential number of extant species is estimated at 30,000. Included in this number are many ectosymbiotic and endosymbiotic species, as well as some obligate and opportunistic parasites. Ciliate species range in size from as little as 10 µm to as much as 4 mm in length, and include some of the most morphologically complex protozoans. In most systems of taxonomy, 'Ciliophora' is ranked as a phylum, under either the kingdom Protista or Protozoa. In some systems of classification, ciliated protozoa are placed within the class 'Ciliata,' (a term which can also refer to a genus of fish). In the taxonomic scheme proposed by the International Society of Protistologists, which eliminates formal rank designations such as 'phylum' and 'class', 'Ciliophora' is an unranked taxon within Alveolata. Unlike most other eukaryotes, ciliates have two different sorts of nuclei: a tiny, diploid micronucleus (the 'generative nucleus,' which carries the germline of the cell), and a large, polyploid macronucleus (the 'vegetative nucleus,' which takes care of general cell regulation, expressing the phenotype of the organism). The latter is generated from the micronucleus by amplification of the genome and heavy editing. The micronucleus passes its genetic material to offspring, but does not express its genes. The macronucleus provides the nuclear RNA for vegetative growth. Division of the macronucleus occurs by amitosis, and the segregation of the chromosomes occurs by a process whose mechanism is unknown. This process is not perfect, and after about 200 generations the cell shows signs of aging. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during conjugation. Here two cells line up, the micronuclei undergo meiosis, some of the haploid daughters are exchanged and then fuse to form new micronuclei and macronuclei. Food vacuoles are formed through phagocytosis and typically follow a particular path through the cell as their contents are digested and broken down by lysosomes so the substances the vacuole contains are then small enough to diffuse through the membrane of the food vacuole into the cell. Anything left in the food vacuole by the time it reaches the cytoproct (anus) is discharged by exocytosis. Most ciliates also have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure, or in some function to maintain ionic balance. In some genera, such as Paramecium, these have a distinctive star shape, with each point being a collecting tube. Cilia are arranged in rows called kineties. In some forms there are also body polykinetids, for instance, among the spirotrichs where they generally form bristles called cirri. More often body cilia are arranged in mono- and dikinetids, which respectively include one and two kinetosomes (basal bodies), each of which may support a cilium. These are arranged into rows called kineties, which run from the anterior to posterior of the cell. The body and oral kinetids make up the infraciliature, an organization unique to the ciliates and important in their classification, and include various fibrils and microtubules involved in coordinating the cilia.